This invention relates to cathode ray tube image displays, and is addressed to means for suppressing AC electric fields that emanate from the viewing screen of such displays. More particularly, the objective is to suppress such emissions to a level below the level of a recognized standard. The invention is applicable to both monochrome and color cathode ray tube image displays in which the viewer may be in close proximity to the faceplate, such as the viewers of television sets and visual display terminals.
The present invention had its origin in the concern over the possible detrimental effects of AC electric field emissions on the physiology of the viewers. Testing for such emissions in visual display terminals is described in a booklet published by the National Board for Measurement and Testing (MPR) of Sweden entitled "Test Methods for Visual Display Units: Visual Ergonomics and Emission Characteristics." MPR 1990:8 1990-1991, Boras, Sweden, pp. 51-56. This standard is known as "MPR-2. "
The frequency range of interest specified in standard MPR-2 is 2 kHz to 400 kHz, in the very low frequency (VLF) region. The standard specifies an electric field level below 2.5 V/m in this region.
The funnel of a cathode ray tube (CRT) has an outer coating and an inner coating, both of which are electrical conductors and which may also be partially electrically resistive. The inner coating is located on the inside surface of the CRT funnel, and is electrically connected to a very thin, electrically conductive film of aluminum that covers the phosphor viewing screen located on the inner surface of the faceplate. The outer coating, located on the outside surface of the funnel, typically has an electrical resistance, known as the "sheet resistance," in the range of 2.times.10.sup.1 to 2.times.10.sup.2 ohms/square. In some applications, the outer coating may have no resistive component and comprise a resistance-free conductor. The inner coating typically has a sheet resistance of 400 ohms/square. In this disclosure, both coatings are electrically resistive. The coatings are made resistive primarily to leak off electrostatic charges which may otherwise build up on the coatings and initiate destructive arcing within the tube envelope.
The outer coating of the funnel is normally connected to ground, while the inner coating is at a electrical potential in the range of fifteen to thirty kilovolts, with the magnitude of the potential depending upon the type of CRT. This high potential, which is known as the ultor voltage, is generated by a high-voltage power supply and is routed to the inner coating by way of an anode button which penetrates the glass of the funnel. An uncoated, bare-glass area around the anode button isolates the high voltage on the anode button from the grounded outer coating.
A capacitor is thus formed which serves as a component of the filter circuit of the power supply, and in conjunction with other power supply components, acts to smooth out peaks in the output of the high-voltage power supply. As both the inner and outer coatings cover extensive areas inside and outside the funnel, respectively, the resulting capacitance is relatively large; that is, in the range of 1,000 to 2,000 picofarads.
The horizontal deflection circuit provides a train of pulses having a frequency of 15,705 Hz in monochrome television sets, a frequency of 15,734.26 Hz in color television sets, and frequencies of up to 150 kHz in some visual display terminals. The pulses are routed to the horizontal winding of a beam-deflection yoke, along with vertical deflection pulses from a vertical deflection circuit. The yoke impels the electron beam or beams to scan a raster on the phosphor viewing screen which is deposited on the inner surface of the faceplate.
Ideally, the ultor voltage is a fixed, constant voltage. However, the pulses of the horizontal deflection circuit are capacitively coupled from the yoke to the inner coating on the funnel, and to the high-voltage power supply that supplies the ultor voltage. As a result, ripples appear on the ultor voltage that induce undesired AC electric field emissions from the viewing screen of tube.
Attempts to reduce AC electric field emissions have included the shielding of the faceplate section of the CRT, and the use of additional filtering components in the horizontal deflection circuit and the high-voltage power supply. Such attempts have not been completely satisfactory in that they have been costly in terms of convenience, components, and performance.